PROJECT SUMMARY/ABSTRACT Co-use of tobacco and cannabis is increasingly prevalent and may be associated with greater physical and psychological risks when compared to use of either substance alone. Co-use is particularly prevalent in young adulthood, an age group where risk for problematic substance use is heightened. Despite growing rates of co-use in this age group, factors that may maintain co-use are less well known. Individual differences in positive affect response to drug use and neural reward reactivity are important predictors of drug use maintenance, but are relatively understudied in this population. Thus, the specific aims of the proposed study are to 1) Characterize subjective positive affect response to cannabis and nicotine co-use, as compared to single- use of either substance 2) Investigate relationships between non-intoxicated neural reward reactivity and positive affect response following episodes of cannabis, nicotine, and co-use, and 3) Examine rates of cannabis and nicotine use in the past month as potential moderators of the relationship between non-intoxicated neural reward reactivity and positive affect change following drug use. To address these aims, the current study proposes to recruit a subset of participants from a large-scale ecological momentary assessment (EMA) study of young adult cannabis and tobacco co-users to participate in an additional electroencephalogram (EEG) laboratory visit. These study aims support the applicant’s training goals to 1) develop knowledge in phenotypes of concurrent substance use through investigating affective, neural, and behavioral correlates of nicotine and cannabis co-use 2) acquire training in EMA methods and data analysis 3) gain training and experience in EEG methods 4) pursue academic professional development through authoring manuscripts, conference presentations, and career development and grant writing seminars and workshops, and 5) gain training in ethics and responsible conduct of research. The applicant’s mentorship team is comprised of experts in young adult nicotine and cannabis use research and EMA methods (Dr. Mermelstein), affective and neural correlates of cannabis use in young adulthood (Dr. Crane), EEG methods and analysis (Dr. Burkhouse), and advanced statistical methods (Dr. Hedeker). The mentoring team will thus advance the applicant’s goals to pursue training in concurrent substance use research, EMA methods, and EEG methods. The proposed training and research plan ultimately supports the candidate’s long-term goal of pursuing a career as an independent investigator dedicated to understanding affective, behavioral, and neural correlates of concurrent substance use in young adulthood. Further, the proposed study has the potential to greatly advance our understanding of correlates of affective and neural correlates co-use in young adulthood that can be harnessed as future prevention and treatment targets.